Buy something used and inexpensive for a starter bike.
Look for something old but not too old. You don't want something so old that it is a junker, but not something so new that you'll feel bad if you lay it down.
Look for something in the 500 - 800 CC engine size. This will be powerful enough so you don't get bored too soon, but not so fast that you will be afraid of it.
Look for a standard or cruiser bike for your first bike. They are more forgiving. Stay away from the sport bikes for your first bike as they require more skill. Also used sport bikes tend to be more abused.
Watch the classified section of your local news paper. Some locations have shopper magazines with used motorcycles in your area. Reasonable used bikes are out there. Be patient. Don't buy too quickly, but if you find one you like, buy it right away. Unfortunately, with the price of gas, the price of used bikes has also risen. If you find a good starter bike, don't hesitate as they can sell quickly.
Don't get hung up on any particular brand. They are all about the same. Pay more attention to the bike's condition. If any one brand were really superior to all other brands, this would be the only brand bike on the road. In addition, if any brand bike was really inferior to the other brands, nobody would buy them and the company would be out of business.
Learn on this bike. Make your mistakes on it. You will make mistakes. Nearly all new riders will lay their bike on its side at least once. Would you rather make your mistakes on an older bike or your nice shiny expensive new bike?
The most important thing you should do when getting your first bike is to take a motorcycle safety course.
After you have been riding a while and gain some experience, you can sell your starter bike and get the one you want. If maintained properly and not abused, you won't lose too much money on your starter bike. Put the money from the sale towards the bike you really want. By this time, you will have figured out what style bike best suits your riding style and needs.
If you settle on a sport bike for your next bike, avoid the used ones. As I said before, these bikes tend to be more abused. So if considering a sport bike, consider something new.
I know I used this answer before. However I feel this is good advice.
Have fun - Ride safe